Recovery of dimethylformamide by crystallization and distillation

ABSTRACT

Solvent recovery process and apparatus for separating pure dimethylformamide from admixture with sodium chloride and water using a first stage of partial evaporation producing salt crystallization and separation and a second stage of fractional distillation using a high volume bottom products flow.

United States Patent 1191 Thomas 1 1 Jan. 30, 1973 [541 RECOVERY OF [56]References Cited DIMETHYLFORMAMIDE BY CRYSTALLIZATION AND UNITED STATESPATENTS DISTILLATIQN 3,231,605 1/1966 Blumbergs ..203/4s 2,773,81512/1956 Nicolaisen ..203/82 lnvemori Edward Tlwmasi 2,640,017 5/1953Graff ..203/99 Wilmslow, Cheshlre' SK 92 JY, 3,294,051 12/1966 Pasin..260/561 R gland 2,417,862 3/1947 Dale ..203/14 I 4 2,895,886 7/1959[73] England 3,530,043 9/1970 Horn et a1. ..202/155 M 27, 1970 V [22] PMFOREIGN PATENTS OR APPLICATIONS [21] App1.No.: 40,846

715,839 9/1954 Great Britam ..260/561 R [30] Foreign ApplicationPriority Data Primary Examiner-Wilbur L. Bascom b, Jr. 1Attorney-Watson, Cole, Grindle & Watson May 28, 1969 Great Britain.-..26985/69 [57] ABSTRACT Aug. 2*], 1969 Great Britain..... ..41832/69 r1 Solvent recovery process and apparatus for separating [52] U.S. Cl...203/48, 203/14, 203/82, pure dimethylformamide from admixture withsodium 1 203/99, 202/155, 23/303, 260/561 R chloride and water using afirst stage of partial 51 int. Cl. ..C07c 103/36, B01d 9/00 evaporationProducing Salt crystallization and p tion and a second stage offractiona1 distillation using [58] F eld of Search...260/56l R; 202/155,153, 175; a high volume bottom products HOW 203/14, 48,47,,81,82,84,94,D1G. 9; a 159/45; 23/302, 303 5 Claims, 3 Drawing Figures PmmmmaoI915 3.713.991

SHEET 2 or 2 INVENTORS BQ IAN EbwARD ALs-n TuomAS w at; W'AQL ATTORNEYRECOVERY OF DIMETHYLFORMAMIDE BY CRYSTALLIZATION AND DISTILLATION Thisinvention relates to the recovery in substantially pure form of polarorganic solvents from mixtures with one or more liquid componentsmiscible therewith, the mixtures containing at least one dissolved solidcomponent. It is particularly applicable to the recovery of expensivewater soluble polar organic solvents for example acylamides such asN,N'-dimethyl-formamide from aqueous solutions of such solvents whichcontain dissolved solids for example inorganic salts such as chloridesespecially sodium, potassium or, calcium chlorides. It is particularlyuseful in achieving the separationof materials, such asdimethylformamide, which are less volatile than water, though it may beapplicable to any mixture of two liquids from which it is wished torecover the less volatile component in pure form. The invention extendsboth to methods of carrying out the solvent recovery and also toapparatus for carrying out the method.

Certain earlier proposals have suggested that dimethylformamide shouldbe recovered from aqueous solutions containing inorganic salts byextraction of the dimethylformamide with a suitable solvent such asmethylenechloride. However, this process requires the use of a complexplant to effect the subsequent separation of the dimethylformamide fromthe extracting solvent. In addition'some of theexpensive extractingsolvent is lost in the aqueous stream leaving the extraction apparatus.In addition the extraction of the dimethylformamide from therawfeedstock is not essentially complete and therefore the waste aqueousstream con tains small amounts of dimethylformamide in addition. tocontaining small amounts of the extracting. solvent and these amountsare such as to render the effluent unsuitable for discharge direct intowater courses or sewers without further additional treatment which addsto the cost of the total process.

Another proposal has been that the dimethylformamide should be recoveredfrom aqueous solutions containing dissolved inorganic salts by countercurrent stripping a substantial part of the dimethylformamide using opensteam in a stripping column. The dimethylformamide would then besubsequently separated from the waterby distillation. However, thevapor/liquid equilibrium of a dimethylformamide/water mixture 'does notfavor the removal of dimethylformamide in this way exceptin cases. wherethe inorganic salts possess the property of effectively-reducing thevapor pressure of the water componentof the mixturebelow that of thedimethylformamide. Even in cases where the salt can have this effect ithasto be'present in a concentration sufficientlyhighto bring about therequired reduction in vapor pressure of'the water component. The

process is thus notgenerally applicable; In addition the aqueous residuefrom the stripping. column again is liable to containsomedimethylformamide and this may be such as to render the effluentunsuitable for discharge into a water course or-sewer without furthertreatment which once more willraise the cost of the total process.

It isan object of the present invention to provide a processfortheseparation of organic liquids from aqueous solutionscontaining dissolvedsolidswhich is more generally applicable than the known processes andwhich is also cheaper. It is a further object to provide a processwhereby a higher proportion of the original organic solvent in thefeedstock can be recovered and the effluent from the process containssufficiently low amounts, if any of the organic liquid to render theaqueous effluent acceptable for direct discharge into water courses orsewers without further treatment. It is yet another object of theinvention to provide a process whereby the recovered organic liquid isof high purity enabling it to be reused in the process from which it wasoriginally an effluent.

According to the present invention a method of recovering a polarorganic solvent, for example N,N- dimethylformamide, from a feedstockcomprising a mixture of the said polar organic solvent and at least oneother liquid component miscible therewith, for example water, the saidmixture containing at least one dissolved solid component, for examplesodium chloride, comprises evaporating off at least part of the liquidcomponents leaving a mother liquor whereby dissolved solid is caused toprecipitate in the mother liquor separating substantially liquor freesolid from the mother liquor, and separating the said. polar organicsolvent from the mixture'of evaporated liquid components, for example byevaporative separation techniques sucha's fractional distillation.

It is intended to describe the invention with reference todimethylformamide, water and sodium chloride but it should beappreciated that it is applicable to other systems as well. Thus organicsolvents other than dimethylformamide could be recovered by the processand the second liquid component need not necessarily be water and couldbe other liquids miscible with the organic liquid component which it isintended to recover. In addition, the dissolved solid component could beother than sodium chloride.

Effluent of this sort are produced by certain processes used in theproduction of microporous polymer sheet materials in particular frompolyurethanes but other solvent systems containing dissolved solidcomponents may be involved with alternative processes. These may besystems in which the first component is a polar solvent such asdimethylformamide, dimethylacetamide or dimethylsulphoxide and thesecondliquid' component may be water though it can be other hydroxylcontaining liquids such as alcohols or 'glycols for example propyleneglycol or ketones such as acetone or similar'materials which have theeffect of reducing the solubility of the polymer inasolution of thepolymer ina solution of the polar organic solvent. The dissolved solidmay be any material which is soluble in the system and which facilitatesor assists in coagulation of the polymer. or the production of an'evenfine pore size. Such-materials include ammonium nitrate and urea. Withsuch materials it will of course be necessary to take particular care asto the temperature conditions involvedto make certain that explosionhazards do notarise.

It will be appreciated thatthe process defined above need notnecessarily be carried out at atmospheric pressure and could easily becarried out at subatmospheric pressures or at super-atmosphericpressures if the particular vapor pressure relationships of thecomponents involved made this desirable. The dissolved solidcomponentprecipitated or crystallized in the mother liquor is separatedtherefrom by any known means for example by centrifuge or filterarrangement. Preferably this separation is achieved by drawing offmother liquor containing precipitated solid and separating the solidfrom entrained liquor which is returned to the main body of motherliquor.

The precipitation can be carried out as a continuous process in the baseof the evaporator and solids containing mother liquor continuouslywithdrawn to a continuously operating centrifuge or the precipitationcan be carried out in a separate salt box separated from the evaporatorand the centrifuge by valves. The precipitation is then a batch process.

In one form of the invention the evaporated liquid components in thevapor phase are fed to a fractionating column or a fractionatingapparatus adapted for use with the particular liquid components involvedand this is preferably run so that the lighter component is separatedfrom the top of the column substantially freed from the heaviercomponent which is received from the bottom of the column. This heaviercomponent may be passed through a re-boiler, the heavy fraction fromthis re-boiler may be passed to a still from which substantiallypurified first component can be evaporated. Traces of impurities ofheavier components and traces of dissolved solid still present in thesystem can be periodically removed from the bottom of the said still.

According to a further aspect of the invention apparatus for carryingout the method in accordance with the first aspect of the inventioncomprises evaporator means provided with heating means, for example anexternal pump circuit including a heating device, and with outlet meansto separative means for separating the evaporated liquid components forexample evaporative separating means and with outlet means enablingsolids produced in the evaporator means to be discharged, for example;under gravity, to solids separating means for separating the solid fromentrained liquid and means for returning the separated liquid to theevaporator means.

Preferably the evaporative separating means is a fractional distillationcolumn desirably provided with reboiling means for returning thevaporized part of the heavier output fraction to the base of the columnand recycling condenser means for returning the portion of the lighterfraction from the column as a reflux flow to the head of the column.

A still and means for passing the heavier fraction of the reboiledoutput from the base of the column may also beprovided. Substantiallypure first component can be evaporated off from the said still and theheavier fractions in the said still either passed to discharge orreturned to the evaporator for example by the same means as theseparated liquid is returned to the evaporator from the solidsseparating means.

The invention can be put into practice in various ways but as mentionedabove will be described with reference to the recovery ofdimethylformamide from an aqueous solution containing dissolved sodiumchloride, and with reference to the accompanying diagrammatic drawingillustrating one specific embodiment of the process and apparatus forcarrying it out, and two modifications thereof.

FIG. 1 is an outline flow diagram ofthe process and a diagrammaticrepresentation of apparatus for carrying out the process.

FIG. 2 is a flow diagram of the first modification of 1 the inventionused to produce a pure water effiuent stream.

FIG. 3 is a flow diagram of the second modification of the inventionused to separate formic acid and involatile impurities when N,N'dimethylformamide is the desired product.

The feedstock material, containing dimethylformamide and the inorganicsalt in aqueous solution, is delivered by a pump 1 via a conduit 2 to anevaporator 3. The evaporator 3 is provided with an external heatingcircuit comprising a calandria 4 and a circulating pump 5. The calandriamay be heated by steam or other suitable medium and the heat addedcauses at least part of the liquid portion of the feedstock to vaporize.The vapors pass from the evaporator 3 by the conduit 6 to the feedplateof a distillation column 7.

As a result of evaporation of the feedstock in the evaporator 3 the saltis precipitated and is discharged in the form of a crystal suspension inthe mother liquor from the evaporator through a conduit 8 to separatingmeans 9, which can be a centrifuge or a filter. Solid salt,substantially freed of liquor, is discharged, via a conduit 10 fordisposal. Means may be provided at the separating means 9 for washingthe solid salt with water, prior to discharge. The other liquorseparated from the salt in the separating means 9 flows via a conduit 11into a tank 12, from which it is returned to the evaporator by a pump13, a conduit 14 and the conduit The vapors entering the fractionaldistillation column 7 from the evaporator 3 pass up the column 7 andleave at the top via a conduit 15 and thence to a condenser 16. Thecondensate from the condenser 16 is divided into two parts, by any knownmeans, one part being returned to the head of the column 7 via a conduit17 as a reflux. The other part of the condensate flows, via a conduit 18to storage or other disposal. This part contains a substantialproportion of the volatile amines present as impurities in thedimethylformamide.

The reflux flows down the column 7 and leaves at the base via a conduit19. Some of this refluxed liquid then flows via a conduit 20 into areboiler 21, where heat is added, by means of steam or any othersuitable medium, as is necessary efflciently to perform the function offractional distillation in the column 7. The vapor and liquid streamfrom the reboiler 21 then passes via a conduit 22 into the base of thecolumn 7.

Recovered dimethylformamide, in quantity approximating to that in thefeedstock is withdrawn via a conduit 23, aided if necessary by a pump 24and delivered to a still 25. In the still 25, heat is added, by steam orany other suitable medium, so as completely to vaporize thedimethylformamide delivered to the still. The resulting vapors pass viaa conduit 26 to a condenser 27. Condensed dimethylformamide flows fromthe condenser 27, via a conduit 28 to storage.

Impurities and materials of low volatility, which collect in the saidstill 25 are discharged either periodically or continuously via theconduit 29 into the mother liquor tank 12. By these means, any solidmaterials, such as salt, tending to accumulate in the still 25 is purgedfrom the system and will eventually be discharged as solid, at theseparating means 9. Provision may also be made for discharging some ofthe residual liquor from the still 25, via a conduit 30 for disposal.

Although described as having an external calandria, the evaporator maybe of any known type, suitable for salting applications.

It will be apparent that the final still 25 can be arranged for eithercontinuous or batch operation.

While it is normally envisaged that the vapors will be passed from theevaporator to the fractional distillation column, in certaincircumstances, it may be convenient fully to condense the said vaporsand subsequently to feed the resulting condensed liquid to thefractional distillation column. In this case, the evaporator may takethe form of a plurality of evaporators operating in series, in multipleeffect.

Where the recovered organic chemical, produced from the final still,contains an impurity of relative volatility lower than that of theorganic chemical, the said organic chemical may be purified by furtherdistillation in a fractional distillation column, according to theestablished techniques of fractional distillation. In.

this event, it will be convenient to take the vapors from the finalstill directly into the said fractional distillation column.

This invention also extends to a modification to the process useful inthe separation of volatile impurities or decomposition products, such asamines, from the water effluent. It is illustrated in FIG. 2. These arevolatile impurities liable to contaminate the water fraction produced bydistillation of aqueous solutions of certain organic water solublesolvents such as dimethylformamide with which the present invention isespecially concerned. When dimethylformamide is brought into contactwith water, some volatile amines, principally dimethylamine, are formedas a result of hydrolysis. In the separation of the water from thedimethylformamide, in a distillation process, the water is the morevolatile componentand passes to the top of the fractional distillationequipment, while the dimethylformamide passes to the bottom. The aminesresulting from hydrolysis are more volatile than water and, if the waterfraction is taken directly from the top of the column, it iscontaminated with the amines. The quantity of amines present in such awater fraction generally will be such as to preclude the directdischarge of the aqueous stream into a water course. Furthermore, thepresence of even relatively small quantities of the amines impart astrong and unpleasant odor to the water making it often unacceptable forre-use as process water. It is possible to remove amines from such awater stream by ion exchange, but the capital and running costs of suchmeasures are not normally at tractive. Moreover, the regeneration of theion exchange medium would require the use of chemical reagents, thedischarge of which would present a further effluent problem. It would bepossible to redistill the amine containing 'water in a furtherdistillation unit, but again, the capital and operating costs would makethis unattractive.

According to this form of the present invention when the mixture ofevaporated liquid components comprises water optionally volatile andN,N'-dimethylformamide a-water stream of effluent purity is removedtherefrom in a fractional distillation apparatus by running the highvolatility region of the apparatus under conditions of high reflux sothat the volatile amines concentrated therein and removing the waterstream as a side stream from a lower volatility region of the apparatusthe N,N' dimethylformamide being removed from the low volatility regionof the apparatus. The apparatus is conveniently a vertical distillationcolumn and then the high volatility region is the top of the column. Theamines would be discharged, as an amine rich solution in water, as partof the condensate stream from the condenser receiving vapors from thehead of the column. Apparatus for carrying out this aspect of thepresent invention comprises fractional distillation apparatus providedwith an additional section at the high volatility region, means forrunning this section at high reflux and output means for withdrawing aside-stream from a region of lower volatility adjacent the said highvolatility section and output means for withdrawing a side stream from alow volatility region. The feedstock is produced by the method andapparatus described in connection with FIG. 1. Referring now to FIG. 2,6 is the feed to a distillation column 7. The vapors pass to the head ofthis column and through a conduit 15 into a condenser 16. The condensatefrom this condenser is divided, the major part returning via the conduit17 to the head of the column as reflux and the small remaining partbeing discharged via the conduit 18, as an amine rich solution in water,for disposal. The water fraction, substantially free of amines, leavesthe column as a side stream, via the conduit 201.

Typical operating conditions for such a plant might be as follows:

The column has six plates above the side-stream conduit 201 and they arerun at a reflux ratio of 159 to 1. An amine rich stream is removed fromconduit 18 at a rate of about 200 lbs/hour, and contains approximately1.0 percent w/w dimethylamine. An effluent purity water side-stream isremoved from conduit 201 at 16,000 lbs/hour and contains about 20 ppm ofdimethylamine. This can either be reused as process water or dischargeduntreated into a sewer.

It was mentioned above that when dimethylformamide is brought intocontact with water some hydrolysis occurs. In addition to producingvolatile amines such hydrolysis also produces involatile acids inparticular formic acid. A further modification of the present inventionrelates to the separation of such involatile impurities from thedimethylformamide. When one attempts to separate the dimethylformamideand water by distillation any formic acid present tends to form theformic acid/dimethylformamide azeotrope, which boils at a temperature ofapproximately 163 C., related to an absolute pressure of 760 mm. ofmercury. The boiling point of dimethylformamide is 153 C., relatedto anabsolute pressure of 760 mm. of mercury. Consequently, the formic acidtends to concentrate, in the form of the azeotrope, at the base of adistillation column and, with an adequate number of plates provided inthe column at sufficient reflux ratio, a substantially puredimethylformamide stream can be removed from the column as a side-streamfrom a point located between the feed plate and the base of the column.

The quantity of formic acid usually passing through the process is smallrelative to the quantity of dimethylformamide. Consequently, because thedimethylformamide product is removed from the distillation column as aside-stream the relative rate of removal of formic acid contaminatedmaterial from the base of the distillation column is very small. Thus,if any non-volatile matter is present in the feed to the distillationcolumn or forms, due to a process of polymerization under heat in thereboiler at the base of the column, the said nonvolatile matter canaccumulate to an extent at which it is precipitated particularly on theheating surfaces of the reboiler. This has the disadvantage that heatingsurfaces can become fouled and thus require cleaning, and, also,conditions conducive to corrosion are established.

It is a known technique, when fractionally distilling materials in whichthe bottoms product from the column is very small in volume, to purgethis material from the column base and subsequently to distill it, toapproaching dryness, in a separate pot still, the object of thisoperation being to recover the maximum quantity of volatile materialfromthe column bottoms product. In this method of operation, it is usualto restrict the quantity of material leaving the column bottom to theminimum, so as to reduce the heating duty on the said pot still. Thevolume of the purge in a conventional system would be of the order ofone-twentieth to one-fiftieth of the actual product flow e.g. of

dimethylformamide. However, we have found that by operating in this way,the concentration of non-volatile material in the base of thedistillation column and associated reboiler is liable to rise to a levelat which precipitation on the heating surfaces of the equipment takesplace.

According to this form of the present invention when the mixture ofevaporated liquid components comprising water N,N'dimethylformamide,involatile impurities and optionally formic acid, anN,N-dimethylformamide stream substantially free from involatileimpurities and formic acid is recovered as a side-stream usingfractional distillation apparatus by removing a liquid bottoms productfrom the distillation apparatus in sufficient quantity to substantiallyavoid precipitation of non-volatile material in the column or associatedheating means, conveying the liquid bottoms product to still means suchas to allow volatile constituents in the bottom product to be vaporizedand returned to the distillation apparatus whilst retaining theremaining bottoms product in a conveniently dischargeable liquidcondition as by the presence of unvaporized volatile materials,vaporizing volatile constituents in the bottoms product and returningthem to the distillation apparatus, preferably to its base, and removingfrom the still means a liquid stream containing the non-volatile matter.It is illustrated in FIG. 3.

The throughput required and composition of the feed-stock will determinethe volume of bottoms product which will have to be removed tosubstantially avoid precipitation but this volume will be relativelyhigh compared with a conventional bottom purge. Thus whilst it ispreferably of the same order as the dimethylformamide flow it may besignificantly lower though it will always be in excess of one-tenth ofthe product flow and preferably in excess of one-half. The volume of thebottoms product will in turn determine the volume and heating capacityof the still means, which can be a pot still of continuous or batchtype.

The pot still is desirably so constructed that fouling of its heatingsurfaces does not readily take place whilst if such fouling should occurit can relatively readily be removed as compared say with fouling in adistillation column or reboiler.

The still means are preferably fitted with mechanical agitator means toimprove heat transfer and to assist in maintaining any precipitatedsolids in suspension.

If desired a number of stills could be provided, arranged so as toprovide standby capability in the event say of one still requiringcleaning.

The feedstock is preferably produced by the method and apparatusdescribed in conjunction with FIG. 1. Referring now to FIG. 3, 301 is adistillation column, with the feed entering the column via a conduit302. The more volatile components in the feed are removed at the head ofthe column 301 in the vapor phase, via the conduit 303, passing to thecondenser 304. The condensate from the said condenser is then dividedinto two portions, one of these returning to the head of the column 301via the conduit 305 and the remainder passing out as overhead product,via the conduit 306. Alternatively the arrangement described inconnection with FIG. 2 could be used.

The dimethylformamide product, leaves as a vapor side-stream via theconduit 307 passing into the condenser 308 and the resulting condensateleaving by the conduit 309.

A reboiler 310 is provided at the base of the distillation column toprovide the heat necessary for distillation. Reflux liquid accumulatingin the base of the column returns to the reboiler 310, via the conduit31 1. Vaporized reflux, from the said reboiler, returns to the base ofthe column, via the conduit 312.

A continuous purge of liquid is taken from the base of the column viathe conduit 313 into the pot still 314. The said pot still is heated byany known means and the feed entering it is substantially vaporized, thevapors returning to the base of the column 301 via the conduit 315.Liquid containing concentrated non-volatile matter, is discharged fromthe pot still 314 via the conduit 316, the said discharge taking placeeither intermittently or continuously according to convenience.

The pot still 314 may be fitted with a mechanically driven agitator 317.

In the conventional operation of such a plant in which the feed to thedistillation column contained 10. w/w dimethylformamide, 1.0 ppm sodiumchloride and the remainder water, since sodium chloride has a solubilityof only about 500 ppm in dimethylformamide, a

conventional purge of one-twentieth to one-fiftieth of thedimethylformamide flow would cause the concentration at the base of thecolumn to be such that the salt would be nearly on the verge ofcrystallizing. If, in accordance with the present invention, the purgeis for example equal to the dimethylformamide flow the concentration ofsalt would only rise to 10 ppm at the base of the column andcrystallization would be extremely unlikely to occur.

A particularly satisfactory installation utilizes the combination of allthe three foregoing features of the invention namely the salt separationof FIG. 1 the amine separation of FIG. 2 and the formic acid andinvolatile material separation of FIG. 3.

This combined process enables a reusable dimethylformamide stream to beproduced and also an aqueous stream which can be discharged to wastewithout further treatment to be recovered from a raw waste product ofsodium chloride, water and dimethylformamide and provides a relativelysimple and uncomplicated process and relatively simple and cheapapparatus for carrying out the process.

The purpose of the purge is to ensure that solids particularly salt donot crystallize out at the bottom of the distillation column. Themaximum concentration 7 of solids which could be allowed to exist at thebase of the column would vary with differing organic solvents and salts.

Thus if the solids concentration at the base of the column can be ashigh as 100 ppm if the input to the column only contains 10 percent DMFand 1.0 ppm of salt the purge would only need to be one-tenth of the DMFproduct flow. If the input to the column contained 5 ppm of salt a purgevolume equal to the DMF product flow would keep the salt concentrationat the base of the column at about 50 ppm and a purge volume of one-halfthe DMF product flow would keep the salt concentration at 100 column.

What I claim as my invention and desire to secure by letters patent is:

l. A method of recovering N,N-dimethylformamide from a feedstockcomprising water and, dissolved in the water, a minor amount ofN,N-dimethylformamide and at least one dissolved water-soluble solid nonvolatile inorganic salt having a low solubility in N,N'-dimethylformamide, the method comprising the steps of (a) feeding thefeedstock continuously to an evaporation zone (b) continuouslyevaporating off part of the N,N-dimethylformamide and water in said zonewhereby to cause said dissolved solid to precipitate so that there isformed a mother liquor containing suspended crystals of said solid (c)drawing off from said zone a portion of said mother liquor containingsuspended crystals of precipitated inorganic solid, (d) mechanicallyseparating said solid from the suspending liquor, (c) then returning theliquor to said zone to mix with the mother liquor therein, (f) leading,as a vapor without condensation, the mixture of N,N-dimethylformamideand water evaporated from said zone to a fractional distillation zonehaving heatingsurfaces at the base thereof, (g) distilling asubstantially pure N,N-dimethylformamide as a side stream from saidfractional distillation zone while, (h) removing from the bottom of saiddistillation zone a stream of liquid impure N,N'-dimethy.lformamidecontaining said inorganic salt dissolved therein and also containingformic acid formed by hydrolysis of said dimethylformamide, thevolumetric rate of removal of said impure stream being above one-tenthand up to 100 percent of the volumetric rate of flow of saidsubstantially pure dimethylformamide side stream (i) conveying saidliquid impure stream to another distillation zone, (j) in said otherdistillation zone, distilling off from said impure stream a vapor streamof volatile constituents and returning the latter stream to saidfractional distillation zone as vapor without condensation whilstretaining in ppm at the bottom of the said other distillation zone adischargable liquid mixture of said inorganic salt and unvaporizedvolatile materials and (k) removing from said other distillation zone aliquid stream containing said inorganic salt.

2. A method as claimed in claim 1 in which said volumetric rate ofremoval of said impure stream is in the range of from above 10 to 50percent of said volumetric rate of flow of said substantially puredimethylformamide side stream.

3. A method as claimed in claim 1 in which the inorganic salt is sodiumchloride.

4. A method as claimed in claim 1 in which the mixture ofN,N'-dimethylformamide and water evaporated from said evaporation zonecontains volatile amines formed by hydrolysis of said dimethylformamideand said volatile amines are distilled off from the head of saidfractional distillation zone and a water stream of effluent purity isdistilled off as an overhead side stream from said fractionaldistillation zone by running the head of the fraction distillation zoneunder conditions of high reflux whereby to cause the volatile amines toconcentrate therein.

5. Apparatus for recovering N,N'-dimethylfo.rmamide from a feedstockcomprising water, and dissolved in the water, a minor amount ofN,N'-dimethylformamide and at least one dissolved water soluble solidnon volatile inorganic salt having a low solubility in N,N'-dimethylformamide which comprises a. evaporator means b. means forfeeding the feedstock continuously to the said evaporator means;

0. means for heating the evaporator means to continuously evaporate offpart of the N,N'-dimethylformamide and water in said evaporator meanswhereby to cause said dissolved solid to precipitate so that there isformed a mother liquor containing suspended crystals of said solid,

d. means for drawing off from the said evaporator means a portion ofsaid mother liquor containing suspended crystals of precipitatedinorganic solid,

e. means for mechanically separating said solid from the suspendedliquor,

f. means for then returning the liquor to the evaporator means to mixwith the mother liquor therein,

g. fractional distillation means having heating surfaces at the basethereof,

h. means for leading, as a vapor without condensation, the mixture ofN,N'-dimethylformamide and water evaporated from the evaporator means tothe said fractional distillation means i. means for distillingsubstantially pure N,N'-

dimethylformamide as a side stream from said fraction distillationapparatus.

j. still means,

k. means for simultaneously removing from the bottom of saiddistillation apparatus a stream of liquid impure N,N'-dimethylformamidecontaining said inorganic salt dissolved therein and also containingformic acid formed by hydrolysis of said dimethylformamide, thevolumetric rate of removal of said impure stream being above one-tenthand up to percent of the volumetric rate of flow of said substantiallypure dimethylformamide side stream,

1. means for conveying said liquid impure stream to said still means,

tion, said stream of vaporized volatile constituents from the stillmeans to the fractional distillation means, and 0. means for removingfrom said still means a liquid stream containing said inorganic salt.

* l l l

1. A method of recovering N,N''-dimethylformamide from a feedstockcomprising water and, dissolved in the water, a minor amount ofN,N''-dimethylformamide and at least one dissolved water-soluble solidnon volatile inorganic salt having a low solubility inN,N''-dimethylformamide, the method comprising the steps of (a) feedingthe feedstock continuously to an evaporation zone (b) continuouslyevaporating off part of the N,N''-dimethylformamide and water in saidzone whereby to cause said dissolved solid to precipitate so that thereis formed a mother liquor containing suspended crystals of said solid(c) drawing off from said zone a portion of said mother liquorcontaining suspended crystals of precipitated inorganic solid, (d)mechanically separating said solid from the suspending liquor, (e) thenreturning the liquor to said zone to mix with the mother liquor therein,(f) leading, as a vapor without condensation, the mixture ofN,N''-dimethylformamide and water evaporated from said zone to afractional distillation zone having heating surfaces at the basethereof, (g) distilling a substantially pure N,N''-dimethylformamide asa side stream from said fractional distillation zone while, (h) removingfrom the bottom of said distillation zone a stream of liquid impureN,N''-dimethylformamide containing said inorganic salt dissolved thereinand also containing formic acid formed by hydrolysis of saiddimethylformamide, the volumetric rate of removal of said impure streambeing above one-tenth and up to 100 percent of the volumetric rate offlow of said substantially pure dimethylformamide side stream (i)conveying said liquid impure stream to another distillation zone, (j) insaid other distillation zone, distilling off from said impure stream avapor stream of volatile constituents and returning the latter stream tosaid fractional distillation zone as vapor without condensation whilstretaining in said other distillation zone a dischargable liquid mixtureof said inorganic salt and unvaporized volatile materials and (k)removing from said other distillation zone a liquid stream containingsaid inorganic salt.
 2. A method as claimed in claim 1 in which saidvolumetric rate of removal of said impure stream is in the range of fromabove 10 to 50 percent of said volumetric rate of flow of saidsubstantially pure dimethylformamide side stream.
 3. A method as claimedin claim 1 in which the inorganic salt is sodium chloride.
 4. A methodas claimed in claim 1 in which the mixture of N,N''-dimethylformamideand water evaporated from said evaporation zone contains volatile aminesformed by hydrolysis of said dimethylformamide and said volatile aminesare distilled off from the head of said fractional distIllation zone anda water stream of effluent purity is distilled off as an overhead sidestream from said fractional distillation zone by running the head of thefraction distillation zone under conditions of high reflux whereby tocause the volatile amines to concentrate therein.